6,6'-Biindeno[1,2-b]fluorene: an open-shell indenofluorene dimer

Area of Science:

• Organic Chemistry
• Materials Science
• Computational Chemistry

Background:

• Indenofluorene (IF) isomers are antiaromatic molecules with varying diradical character.
• Previous studies showed that aryl-substituted [1,2-b]IF derivatives lack experimental diradical properties, contrasting with silyl-ethynyl substituted ones.
• The electronic ground state preference (closed-shell vs. open-shell) of [1,2-b]IF dimers remained an open question.

Purpose of the Study:

• To synthesize and characterize a [1,2-b]IF dimer to investigate its ground state electronic properties.
• To determine if the dimer exhibits closed-shell or open-shell characteristics.
• To explore the influence of dimerization on the diradical character of indenofluorene systems.

Main Methods:

• Synthesis of a 6,6'-biindeno[1,2-b]fluorene derivative, a dimer formed by a C-C single bond linkage.
• Spectroscopic and crystallographic analyses, including single crystal X-ray diffraction.
• Electron paramagnetic resonance (EPR) spectroscopy.
• Quantum chemical calculations to determine diradical character indices (y₀ and y₁) and electronic structure.

Main Results:

• The synthesized dimer exhibited an open-shell bifluorenylidene-type diradicaloid ground state with a diradical character index (y₀) of 0.268.
• The C6-C6' linkage narrowed the HOMO-LUMO gap and induced two Clar sextets, avoiding antiaromaticity.
• Experimental evidence from X-ray diffraction and EPR, supported by theoretical calculations, confirmed the diradicaloid nature.
• A minor tetraradical character index (y₁ = 0.007) was also observed.

Conclusions:

• The [1,2-b]IF dimer preferentially adopts an open-shell diradicaloid ground state.
• Dimerization and strategic C-C linkage can tune the electronic properties of indenofluorene systems, promoting diradical character.
• The study provides crucial insights into the structure-property relationships of antiaromatic and diradicaloid molecules.